Hair and blood samples from the emergency workers who rushed to the scene of last month's attack in Damascus given independently to the United States have shown signs of the powerful sarin nerve gas, Kerry told US television channels.

"In the last 24 hours, we have learned through samples that were provided to the United States and that have now been tested from first responders in East Damascus, (that) hair samples and blood samples have tested positive for signatures of sarin," Kerry told NBC's Meet the Press.

He blitzed the Sunday morning talk shows, upping the ante in the Obama administration's push to build the case for US military strikes against the regime of President Bashar al-Assad.

But after an emergency classifed briefing on Sunday at the US Capitol, many US lawmakers still appeared unconvinced.

Obama, who later this week attends a G20 summit in Russia where Syria is likely to top the agenda, took a huge political risk in handing the decision to deeply-divided lawmakers -- especially after the British parliament voted against any military involvement.

"There's no support for the resolution as it is," said House Democrat Jim Himes after the briefing in reference to the White House's formal request for authorization to conduct strikes.

"There is a lot of concern that the resolution as drafted is overly broad, it has no limitation in either scope or time or activity," he added.

He pointed to the fact that there was "no prohibition in the resolution on putting American troops on the ground" as Obama has promised.

Obama said he had decided the August 21 chemical weapons attack on a Damascus suburb that Washington says killed more than 1,400 people was so heinous -- and such a threat to long-term US security -- that he would respond with a limited military strike.

But he said he believed it was important to win Congress's support when it officially returns from its summer break on September 9.

"If we don't take action now, we send a message to every thug and despot across the world that they can use poison gas on their people, they can commit other war crimes against their people, and there's no price to pay," said Democratic lawmaker Eliot Engel.

But Ileana Ros-Lehtinen, a Republican member of the House Foreign Affairs Committee, said she was "skeptical of what the objectives are."

"What worries me most, as atrocious as this humanitarian, terrible tragedy is in Syria, is what happens next with Iran having nuclear breakout capabilities at any moment," she said.

"If they think that the president is bluffing when he says: this is the red line, then is the president bluffing when he says all options are on the table with regards to Iran?"

The Senate Foreign Relations Committee will debate Syria on Tuesday and a Senate aide told AFP the committee could vote on military force as early as Wednesday.

But influential Republican Senator John McCain, who was due to meet Obama on Monday, said he was not yet sure if he would support the move during a full Senate vote expected later in the month.

"We're in a bit of a dilemma here because I think Senator Lindsey Graham and I, and others, will be wanting a strategy, a plan, rather than just we're going to launch some cruise missiles and that's it," he told CBS television.

"But I also am aware of the failure of Congress to endorse this plan, the signal that it sends to the world, in a very dangerous world, where we've also lost enormous credibility."

He argued any US action needed "room for us to provide assistance to those who are struggling against overwhelming odds right now" to help end a brutal war that has claimed more than 100,000 lives since March 2010.

Republican Senator Rand Paul also delivered a sharp warning that Congress may reject Obama's call for military action amid fears the war could "escalate out of control."

"It's at least 50-50 whether the House will vote down involvement in the Syrian war," he told ABC.

But Kerry insisted Congress would swing behind the president, warning "the stakes are too high here."

"If the United States is unwilling to lead a coalition of people who are prepared to stand up for the international norm with respect to chemical weapons that's been in place since 1925, if we are unwilling to do that, we will be granting a blanket license to Assad to continue to gas," he told ABC.

Retreating from US stands against chemical and nuclear weapons would send "a terrible message to the North Koreans, Iranians and others," he added.